Various types of illuminated signs and markers are known, inter alia for use in emergency situations. Such markers include exit signs, directional markings leading to exits and markers indicating critical items or locations. Emergency electrical lighting, such as battery powered lighting, has been used for this purpose, as have tritium illuminated markers, although they provide a much lower light level. Phosphorescent markers are also known, although many of the applications thereof are in novelty items.
Electrically illuminated markings have the advantage that they shine brightly when power is available. They have the following disadvantages: They require wiring, bulbs and batteries for emergency operation. Installation is relatively expensive and continual maintenance and monitoring is required. Electrically illuminated markings are not readily retrofitted in existing installations and equipment.
Tritium illuminated devices have the advantage that they provide constant, no-maintenance, generally uniform level of illumination. They have disadvantages in that they provide a relatively low level of light, so low that they may not be readily seen during the first moments when a person is plunged into darkness and the person's eyes have not yet become adapted to the darkness.
Phosphorescent devices have the advantage that they can be conveniently located and require no power source, other than exposure to light. They provide a relatively high level of illumination immediately following exposure to light. They have the disadvantage that the level of illumination falls off very quickly, such that they become totally useless within 5-20 minutes after exposure to light, depending on the materials employed.